The present invention relates to a presensitized printing plate for waterless planographic printing, in particular offset printing, and to a process for preparing a printing form by exposure and development of said printing plate. The printing plate of this invention comprises a layer support, a radiation-sensitive layer and an outer, ink-repellent layer of silicone rubber.
Printing plates as described above and comprising positive-working or negative-working radiation-sensitive, in particular photosensitive, layers have, for example, been disclosed by U.S. Pat. No. 3,511,178. Diazonium salt condensation products are employed as negative-working, i.e., photocurable photosensitive compounds. The exposed printing plate is developed by means of a solution in which the unexposed portions of the -photosensitive layer are dissolved and the silicone rubber layer is swollen. The silicone rubber portions on top of the soluble layer portions can be removed by rubbing. The printing form obtained is ink-receptive in the bared areas of the layer support, whereas the silicone rubber layer still present in the non-exposed areas of the layer support is ink-repellent. Thus, the printing form prints a negative image of the transparency employed for exposure Insoluble precipitation products of diazonium salts and phosphotungstic acid are used as positive-working photosensitive compounds. In this case, an intermediate layer comprising a diazonium salt/formaldehyde condensate is provided between the photosensitive layer and the silicone rubber layer, in order to anchor the latter to the photosensitive layer. This intermediate layer is crosslinked by heat when the silicone elastomer layer is cured into the silicone rubber layer and then effects the desired improved adhesion.
There are also known planographic printing plates suitable for waterless printing, wherein other layers which are rendered soluble by exposure are used. Practically all of these plates require an appropriate adhesive layer.
According to German Offenlegungsschrift No. 29 43 379 (U.S. Pat. No. 4,358,522), a photosensitive layer of an o-quinonediazide and a novolak is coated with an intermediate layer of an adhesion-promoting aminoalkyl-alkoxysilane.
German Offenlegungsschrift No. 30 45 979 (U.S. Pat. No. 4,342,820) describes a combination of a particular o-quinonediazide layer and a silicone rubber layer. To improve the adhesion between these layers, silicones or silanes can be added to either one of these layers, or an adhesive layer comprising compounds of this type can be applied between the two layers.
European Patent Application No. 100,938 describes a process for improving the adhesion of corresponding layers by a short, slight overall exposure and treatment with a base, for example, a primary amine. An intermediate layer of aminosilanes and/or organotitanates may additionally be applied.
European Patent Application No. 154,980 describes a similar, presensitized printing plate wherein the photosensitive o-quinonediazide layer additionally comprises a coupling component which in a basic medium, such as that used for development, undergoes a coupling reaction with the non-decomposed diazo compound in the unexposed areas, whereby a relatively insoluble azo dye is formed. At the same time, the silicone layer is more firmly anchored to the quinonediazide layer during the coupling reaction. Additionally, an adhesive layer, for example, of aminosilanes or organotitanium compounds, may be present between the aforementioned layers.
German Offenlegungschrift No. 23 50 211 (British Patent No. 1,442,374) describes, inter alia, a printing plate having a photocrosslinkable layer, a silicone adhesive layer and an ink-repellent silicone rubber layer.
German Offenlegungsschriften No. 23 57 871 (British Patent No. 1,444,381) and No. 23 59 102 describe printing plates for waterless offset printing in which an excess of silane is added to a one-component silicone elastomer layer to improve its adhesion to a photopolymer layer or diazonium salt polycondensate layer.
The references show that adhesive layers or adhesion-promoting additives are used in positive- and negative-working photosensitive layers, in combination with silicone elastomer layers. They are particularly necessary when positive-working layers, i.e., layers which are rendered soluble by exposure, are employed, since in these cases no additional anchoring of the image areas of the layer, as a result of photocuring or photocrosslinking, takes place.
Low molecular-weight organic silicon compounds, in particular silanes, are preferably used as adhesion-promoting compounds. The vapors of these compounds are injurious to health and therefore have to be disposed of carefully during the plate manufacture. However, in many instances, the adhesion cannot be sufficiently improved by the addition of these compounds. This is particularly true of the adhesion to layers which are rendered soluble by exposure.
The simultaneously filed Application Ser. No. 087,619, filed Aug. 20, 1987 (German Application No. P 36 28 720.2) assigned to the assignee of the present invention and incorporated herein by reference relates to a presensitized printing plate for waterless planographic printing, which comprises a layer support, a radiation-sensitive layer rendered soluble by exposure, an outer, ink-repellent, crosslinked silicone elastomer layer and an intermediate layer by which the adhesion of the silicone elastomer layer to the radiation-sensitive layer is improved. The radiation-sensitive layer comprises, as the essential constituents:
(a) a compound possessing at least one C--O--C bond which can be split by acid,
(b) a compound forming a strong acid upon irradiation, and
(c) a water-insoluble binder.
The intermediate layer comprises amorphous silicic acid.